This invention relates to a tuning system which is suited for a receiver such as a television receiver or RF transceiver.
A conventional tuning system such as disclosed in U.S. Pat. No. 4,142,158 is equipped with a keyboard for selecting a desired channel, a memory for storing a plurality of local oscillation frequencies corresponding to the number of channels and a so-called phase locked loop for maintaining a local oscillation frequency at a predetermined value. When a desired channel is designated by depressing a key on the keyboard, the data for the local oscillation frequency corresponding to the desired channel is read out from the proper memory and a frequency dividing ratio of a programmable divider in the phase locked loop (hereinafter referred to as a "PLL") is determined on the basis of this data. In controlling the oscillation frequency of a voltage controlled oscillator (hereinafter referred to as a "VCO") forming the local oscillator of a tuner, the programmable divider is operable to determine the oscillation frequency. The oscillation frequency from the VCO is applied to the programmable divider where its frequency is reduced, the resulting signal being fed to a frequency comparator. The frequency comparator receives the output from a quartz oscillator as a reference signal and, upon comparing both frequencies with each other, produces such an output signal that renders the oscillation frequency of the VCO constant. The output from this frequency comparator is applied through a lowpass filter to the VCO, from which a predetermined local oscillation frequency correspondng to a desired channel is supplied to the mixer of the tuner. Thus, the channel designated by the keyboard is received.
In the receiving system having the above-described construction, it is desired that the system also be equipped with such a function as will allow a channel to be received by automatically jumping over those channels in which no broadcasting is effected. Depending on a broadcasting area, a station may shift considerably the center frequency of a given channel, even for a channel of the same channel number, in order to prevent interference. In such a case, too, it is desired that the system be capable of being tuned exactly to a selected frequency. When the tuner is of a double super-heterodyne type, further, an automatic frequency control circuit must be added to a second local oscillator so as to cope with the frequency fluctuation of the second local oscillator, if any, even if a first local oscillator is controlled by means of the PLL. Preferably, this problem should also be solved.